1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical network, and more particularly, to a bi-directional optical add/drop multiplexer for performing channel adding/dropping of multiplexed optical signals traveling in a forward or backward direction in an optical network.
2. Description of the Related Art
As WDM technology for transmitting a plurality of channels having different wavelengths from each other through a single-cored optical fiber became commercialized, it has made it possible to transmit a high-capacity optical signal at a high speed. Moreover, development of optical device technology has made it possible to route or switch optical signals and/or to add or drop optical signals in an optical fashion, and thereby it is possible to establish an optical network based on WDM technology.
Typically, an add/drop multiplexer includes a pair of wavelength division multiplexers and a plurality of optical switches. The wavelength division multiplexer may, for example, make use of an arrayed-waveguide grating (AWG) having an extensible channel, simple control and excellent integration. The optical switch may make much use of a 2×2 optical space switch or an optical fiber Bragg grating (FGB) having a wavelength dependency, as an example.
FIG. 1 shows a construction of the conventional bi-directional optical add/drop multiplexer. The optical add/drop multiplexer 100 includes two circulators 120,130, an 8×8 AWG 150, six optical switches 141 to 146, two optical band pass filters (OBPFs) 160,170. The first and second circulators, the 8×8 AWG, and the six optical switches all include a plurality of ports. Herein, if a reference numeral of the corresponding element is given as “xxx”, an nth port of the element will be indicated by a reference numeral of “xxxn” (or “xxxn′”), where n is a natural number. Particularly, when the reference numeral is represented as “xxxn′”, it means that the nth port is located on the right side. The first circulator 120 includes three ports 1201 to 1203, which are designed to output optical signals input into one, i.e., an upper port or an adjacent lower port. The first circulator 120 outputs forward optical signals, input into the second port 1202, to the third port 1203. While it outputs backward optical signals, input from the first port 1201, to the second port 1202. The second port 1202 of the first circulator 120 is connected with an optical fiber 110 for transmitting multiplexed optical signals. The forward optical signals include three channels λ1 to λ3, which have different wavelengths from each other. The backward optical signals also include three channels λ5 to λ7, which have different wavelengths from each other.
The second circulator 130 includes three ports 1301 to 1303, which are designed to output optical signals input into one, i.e., an upper port or an adjacent lower port. The second circulator 130 outputs backward optical signals, input into the second port 1302, to the third port 1303, while it outputs forward optical signals, input from the first port 1301, to the second port 1302. The second port 1302 of the second circulator 130 is connected with an optical fiber 110 for transmitting multiplexed optical signals.
The AWG 150 is provided with eight left-side ports 1501 to 1508 on one end, and with eight right-side ports 1501′ to 1508′ on the other end. The fourth and eighth left- and right-sided ports 1504 and 1508; and 1504′ and 1508′ each function as a path for multiplexed optical signals. The first to third left-side ports 1501 to 1503, the fifth to seventh left-side ports 1505 to 1507, the first to third right-side ports 1501′ to 1503′, and the fifth to seventh right-side ports 1505′ to 1507′ each function as a path for demultiplexed optical signals. In the AWG 150, the fourth left-side port 1504 is connected with the third port 1203 of the first circulator 120, the fourth right-side port 1504′ is connected with the third port 1303 of the second circulator 130.
During operation, the AWG 150 demultiplexes forward optical signals input into the fourth left-side port 1504 and outputs the demultiplexed three channels λ1 to λ3 to the three right-side ports 1501′ to 1503′. The AWG 150 then multiplexes the demultiplexed three channels λ1 to λ3 input into the fifth to seventh right-side ports 1505′ to 1507′ and output the multiplexed forward optical signals to the eighth left-side port 1508. Further, the AWG 150 demultiplexes backward optical signals input into the fourth right-side port 1504′ and outputs the demultiplexed fifth to seventh channels λ5 to λ7 to the fifth to seventh left-side ports 1505 to 1507. The AWG 150 then multiplexes the demultiplexed fifth to seventh channels λ5 to λ7 input into the three left-side ports 1501 to 1503 and to output the multiplexed backward optical signals to the eighth right-side port 1508′.
The first and sixth optical switches 141 to 146 are each provided with first and second ports 1411 and 1412; 1421 and 1422; 1431 and 1432; 1441 and 1442; 1451 and 1452; and 1461 and 1462, which are disposed on one side of each optical switch. The first and sixth optical switches 141 to 146 are each also provided with third and fourth ports 1413 and 1414; 1423 and 1424; 1433 and 1434; 1443 and 1444; 1453 and 1454; and 1463 and 1464, which are disposed on the other side of each optical switch. In a bar (or, parallel) state, the first and third ports 1411 and 1413; 1421 and 1423; 1431 and 1433; 1441 and 1443; 1451 and 1453; and 1461 and 1463 are connected in pairs, and the second and fourth ports 1412 and 1414; 1422 and 1424; 1432 and 1434; 1442 and 1444; 1452 and 1454; and 1462 and 1464 are also connected in pairs. In contrast, in a cross state, the first and fourth ports 1411 and 1414; 1421 and 1424; 1431 and 1434; 1441 and 1444; 1451 and 1454; and 1461 and 1464 are connected in pairs, and the second and third ports 1412 and 1413; 1422 and 1423; 1432 and 1433; 1442 and 1443; 1452 and 1453; and 1462 and 1463 are also connected in pairs. The first optical switch 141 is connected to the first and fifth right-sided ports 1501′ and 1505′ of the AWG 150. The second optical switch 142 is connected to the second and sixth right-side ports 1502′ and 1506′ of the AWG 150, the third optical switch 143 is connected to the third and seventh right-side ports 1503′ and 1507′ of the AWG 150, the fourth optical switch 144 is connected to the first and fifth left-side ports 1501 and 1505 of the AWG 150, the fifth optical switch 145 is connected to the second and sixth left-side ports 1502 and 1506 of the AWG 150, the sixth optical switch 146 is connected to the third and seventh left-side ports 1503 and 1507 of the AWG 150.
The first OBPF 160 has a preset wavelength pass band including the wavelength band of the forward optical signals to remove noise. The first OBPF 160 is connected with the eighth left-side port 1508 of the AWG 150 and the first port 1301 of the second circulator 130.
The second OBPF 170 has a preset wavelength pass band including the wavelength band of the backward optical signals to remove noise. The second OBPF 170 is connected with the eighth right-side port 1508′ of the AWG 150 and the first port 1201 of the first circulator 120.
Hereinafter, a description will be made, for example, regarding a first case where the optical add/drop multiplexer 100 drops the first channel λ1 from the forward optical signals including the first to third channels λ1 to λ3, and a second case where the optical add/drop multiplexer 100 adds the fifth channel λ5 to the backward optical signals including the sixth and seventh channels λ6 and λ7.
A controlling unit (not shown) controls the first and fourth optical switches 141 and 144 into a cross state. Further, it controls the other optical switches 142, 143, 145 and 146 into a bar state. First, in terms of the first case, forward optical signals input into the second port 1202 of the first circulator 120 are output to the third port 1203. The AWG 150 demultiplexes the forward optical signals input into the fourth left-side port 1504 and outputs the demultiplexed first to third channels λ1 to λ3 to the first to third right-sided ports 1501′ to 1503′. The first optical switch 141 outputs the first channel λ1 input into the second port 1412 to split the first channel λ1. The second and third optical switches 142 and 143 output the second and third channels λ2 and λ3, input into the second ports 1422 and 1432, to the fourth ports 1424 and 1434. The AWG 150 multiplexes the second and third channels λ2 and λ3 input into the sixth and seventh right-side ports 1506′ and 1507′ and output the multiplexed forward optical signals to the eighth left-side port 1508. The forward optical signals passing through the first OBPF 160 are input into the first port 1301 of the second circulator 130. The second circulator 130 outputs the forward optical signals input into the first port 1301 to the second port 1302.
Next, in terms of the second case, backward optical signals input into the second port 1302 of the second circulator 130 are output to the third port 1303. The AWG 150 demultiplexes the backward optical signals input into the fourth right-side port 1504′ and outputs the demultiplexed sixth and seventh channels λ6 and λ7 to the sixth and seventh left-side ports 1506 and 1507. The fourth optical switch 144 outputs the fifth channel λ5 input into the second port 1442 to the fourth part 1444. The fifth and sixth optical switches 145 and 146 output the sixth and seventh channels λ6 and λ7, which are input into the second ports 1452 and 1462, to the fourth ports 1454 and 1464. The AWG 150 multiplexes the fifth to seventh channels λ5 to λ7, which are input into the first to third left-side ports 1501 to 1503, and output the multiplexed backward optical signals to the eighth right-side port 1508′. The backward optical signals passing through the second OBPF 170 are input into the first port 1201 of the first circulator 120. The first circulator 120 outputs the backward optical signals inputt into the first port 1201 to the second port 1202.
As mentioned above, the conventional optical add/drop multiplexer employing the N×N AWG takes a fold-back form in which (n−2) numeral ports on the opposite sides are connected to each other in pairs, and makes use of two ports on the opposite sides as a path for multiplexed optical signals. Therefore, there is a problem in that the number of channels available in reality is restricted to (n−2).